P-type thermoelectric materials, a process for their manufacture and uses thereof

ABSTRACT

A thermoelectric material of the p-type having the stoichiometric formula Zn 4 Sb 3 , wherein part of the Zn atoms optionally being substituted by one or more elements selected from the group comprising Sn, Mg, Pb and the transition metals in a total amount of 20 mol % or less in relation to the Zn atoms is provided by a process involving zone-melting of a an arrangement comprising an interphase between a “stoichiometric” material having the desired composition and a “non-stoichiometric” material having a composition deviating from the desired composition. The thermoelectric materials obtained exhibit excellent figure of merits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of priorityto U.S. patent application Ser. No. 11/921,347, filed Feb. 12, 2008, nowU.S. Pat. No. 8,003,002, issued on Aug. 23, 2011, which is a NationalPhase application of and claims the benefit of priority to InternationalApplication PCT/DK2006/000305, filed May 31, 2006, which designated theUnited States and was published in English, and claims priority to U.S.Provisional Application No. 60/686,240, filed Jun. 1, 2005, and EuropeanPatent Application No. 05011707.6, filed May 31, 2005. The disclosuresof all of the aforementioned applications are hereby expresslyincorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to thermoelectric materials of the p-typehaving the stoichiometric formula Zn₄Sb₃, wherein part of the Zn atomsoptionally being substituted by one or more elements selected from thegroup comprising Mg, Sn, Pb and the transition metals.

Furthermore the present invention relates to processes for themanufacture of such improved thermoelectric materials, use of suchthermoelectric materials for the manufacture of thermocouples,thermocouples comprising such thermoelectric materials, use ofthermocouples for the manufacture of a thermoelectric device,thermoelectric devices comprising such thermocouples and uses of suchdevices for thermoelectric purposes.

BACKGROUND ART

Thermoelectric materials have been known for decades. By arranging aso-called p-type thermoelectric material and a so-called n-typethermoelectric material in couples, termed thermocouples, it is possibleto convert heat into electric power or to create a temperature gradientby applying electric power.

A thermocouple accordingly comprises a p-type thermoelectric materialand a n-type thermoelectric material electrically connected so as toform an electric circuit. By applying a temperature gradient to thiscircuit an electric current will flow in the circuit making such athermocouple a power source.

Alternatively electric current may be applied to the circuit resultingin one side of the thermocouple being heated and the other side of thethermocouple being cooled. In such a set-up the circuit accordinglyfunctions as a device which is able to create a temperature gradient byapplying electrical power.

The physical principles involved in these above phenomena are theSeebeck effect and the Peltier effect respectively.

In order to evaluate the efficiency of a thermoelectric material adimensionless coefficient is introduced. This coefficient, the figure ofmerit, ZT is defined as:ZT=S ² σT/κ,

-   -   wherein S is the Seebeck coefficient, σ is the electrical        conductivity, T is the absolute temperature, and κ is the        thermal conductivity. The figure of merit, ZT is thus related to        the coupling between electrical and thermal effects in a        material; a high figure of merit of a thermoelectric material        corresponds to an efficient thermoelectric material and vice        versa.

The techniques relating to the manufacture of thermocouples fromthermoelectric materials as well as the manufacture of thermoelectricdevices from such thermocouples are well documented in the art. See forexample Thermoelectric Handbook (ed. Rowe, M.), CRC Press, Boca Raton,1995 and Thermoelectrics—Basic Principles and new MaterialsDevelopments, Springer Verlag, Berlin, 2001, which are hereby includedas references.

Traditionally thermoelectric materials have been composed of alloys,such as Bi₂Te₃, PbTe, BiSb and SiGe. These materials have a figure ofmerit of approximately ZT=1 and operate at temperatures of 200 to 1300K.

Further improvements appeared with the introduction of alloys of theTe—Ag—Ge—Sb (TAGS) type which exhibit ZT-values of approximately 1.2 inthe temperature range of 670-720 K.

Recently new types of materials were made with alloys of the Zn₄Sb₃type. Caillat et al. in U.S. Pat. No. 6,458,319 B1 disclose p-typethermoelectric materials of the formula Zn_(4-x)A_(x)Sb_(3-y)B_(y),wherein 0≦x≦4, A is a transition metal, B is a pnicogen, and 0≦y≦3. Thematerials are disclosed as being single phased hexagonal rhombohedral.The thermoelectric materials were manufactured as a single crystalprepared in accordance with a gradient freeze technique using a BridgmanTwo-Zone furnace.

By this method however the material obtained tends to containmacro-cracks originating from the cooling of the material. Alternativelya “single phase”, polycrystalline material was obtained using a powdermetallurgy method in which the metals are reacted in a sealed ampoule atelevated temperature whereafter the resulting powder was hot-pressed at20,000 psi and 350° C. The materials exhibit acceptable high figures ofmerit. For example U.S. Pat. No. 6,458,319 B1 discloses, that a ZT of 1at a temperature of 250° C. could be obtained for Zn₄Sb₃ (cf. column 11,lines 13-16). Alternatively, the Zn₄Sb₃-type materials may be preparedby a quench method wherein the elements making up the composition aremelted in an ampoule for 2 hours at approximately 750° C. followed byquenching in water and hot-pressing (cf. Caillat et al., J. Phys. Chem.Solids, Vol. 58, No 7, pp. 1119-1125, 1997.

The known thermoelectric materials of the composition Zn₄Sb₃, in whichpart of the Zn atoms optionally has been substituted by other dopantatoms, however has the disadvantage, that although initial high figureof merits can be obtained, these figure of merits cannot be maintainedat the same level when the material is repeatedly subjected to anincrease and decrease of the surrounding temperature. That is, if thethermoelectric material is thermally cycled, i.e. repeatedly subjectedto an increase and decrease of the surrounding temperature, whichinevitably will happen when used in thermocouples, the figure of meritwill decrease with each cycle until its reach an essential stable valuewhich is considerably lower that the initial value obtained.

This fact is also confirmed in U.S. Pat. No. 6,458,319 B1, in which itis stated that at temperature above 250° C., some decomposition occurredleading to the formation of a ZnSb crystal structure in the samples (cf.column 10, lines 17-21). Once a decomposition to ZnSb has occurred in apart of the material, the material has lost some efficiency in terms ofthe figure of merit. The presence of a ZnSb phase in the material willfurthermore during thermal cycling make the remaining correct Zn₄Sb₃phase more prone to decomposition to the undesired ZnSb phase, becausethe ZnSb phase already present may act as “crystal seeds” for furtherdecomposition. In any event, once decomposition has occurred withaccompanying “loss” of figure of merit, the original figure of meritcannot be re-established and during thermal cycling it is inevitablythat the figure of merit will continue decreasing until an essentialconstant value is obtained.

The effect of ZnSb impurities has been studied by L. T. Zhang et al. (J.Alloys and Compounds 2003, 358, 252-256, “Effects of ZnSb and Zninclusions on the thermoelectric properties of β-Zn₄Sb₃”) and theyconclude that ZnSb and Zn impurities degrade the thermoelectricproperties. In particular it is stated in this document that: “contraryto a previous paper [T. Caillat et al., J. Phys. Chem. Solids 58 (7)(1997), 1119], β-Zn₄Sb₃ was found to be not so stable under vacuum whenheated to high temperatures, mainly because of Zn evaporation”, (squarebracket being added by Applicant), cf. L. T. Zhang et al. J. Alloys andCompounds 2003, 358, 252-256, page 253, paragraph 3.2, line 1).

Hence, it is evident, that the prior art Zn₄Sb₃ materials are not stablewhen subjected to thermal cycling.

Accordingly, a need for further improved thermoelectric materials of thecomposition Zn₄Sb₃, in which part of the Zn atoms optionally has beensubstituted by other dopant atoms and for which the decrease in thefigure of merit during thermal cycling, is reduced, still exists.

Thus it is an object according to one aspect of the present invention toprovide improved p-type thermoelectric materials having thestoichiometric formula Zn₄Sb₃, wherein part of the Zn atoms optionallybeing substituted by one or more elements selected from the groupcomprising Sn, Mg, Pb and the transition metals, and wherein thethermoelectric materials exhibit a high degree of phase purity.

A further object according to a second aspect of the present inventionis to provide a process for the manufacture of such improvedthermoelectric materials and to provide a method for the phasepurification of an already existing thermoelectric material.

Another object according to a third aspect of the present invention isthe use of such thermoelectric materials for the manufacture ofthermocouples.

Yet another object according to a fourth aspect of the present inventionis the provision of such thermocouples.

Still another object according to a fifth aspect of the presentinvention is the use of such thermocouples for the manufacture ofthermoelectric devices.

Yet a still further object according to a sixth aspect of the presentinvention is the provision of such thermoelectric devices.

Finally as an eighth aspect of the present invention is the use of theabove devices for thermoelectric purposes.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a thermoelectric material of the p-typehaving the stoichiometric formula Zn₄Sb₃, wherein part of the Zn atomsoptionally being substituted by one or more elements selected from thegroup comprising Sn, Mg, Pb and the transition metals in a total amountof 20 mol % or less in relation to the Zn atoms; wherein said materialin respect of ZT quality as expressed by the figure of merit, ZT, beingstable after thermal cycling; and wherein said material exhibits afigure of merit, ZT of 0.5 or higher at 350° C. and/or of 0.6 or higherat 400° C.

The present invention also relates to a thermoelectric material of thep-type having the stoichiometric formula Zn₄Sb₃, wherein part of the Znatoms optionally being substituted by Mg and/or Pb in a total amount of20 mol % or less in relation to the Zn atoms.

Furthermore, the present invention relates to a process for themanufacture of a thermoelectric material according to the presentinvention comprising the steps:

i) arranging a rod of a “non-stoichiometric” composition consisting ofZn and Sb and one or two “feeding rods” having a composition accordingto the material of claim 1 in such a way that at least one interface isformed between said “non-stoichiometric” composition and said “feedingrod(s)”, thereby forming an arrangement composed by said rods;

ii) placing the arrangement obtained in step i) in an enclosure andclosing and preferably also evacuating said enclosure thereby forming anampoule;

iii) placing the ampoule obtained in step ii) in a furnace, such as aninduction furnace in such a way that a heating zone is positioned nearthe rod of the “non-stoichiometric” composition;

iv) heating the rod of the “non-stoichiometric” composition in order tostart melting said rod, thereby forming a melting zone;

v) moving the arrangement relative to the heating zone in order to movethe position of the melting zone of the rod arrangement in a directiontowards a “feeding rod”;

vi) allowing the arrangement to cool;

vii) cutting off the part(s) of the arrangement originating from the“feeding rod(s)”, and grinding and hot-pressing the remaining part.

The present invention furthermore relates to a method for the phasepurification of an already existing thermoelectric material and it alsorelates to uses of a thermoelectric material according to the presentinvention for the manufacture of thermocouples, such thermocouplescomprising one or more p-type thermoelectric materials according to thepresent invention, uses of such thermocouples for the manufacture ofthermoelectric devices, such thermoelectric devises and uses suchthermoelectric devices for thermoelectric purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows the effect on Seebeck coefficient during thermal cyclingof a thermoelectric Zn₄Sb₃ material made by a process in analogy withthe prior art quench method. FIG. 1 b shows the effect on electricconductivity during thermal cycling of a thermoelectric Zn₄Sb₃ materialmade by a process in analogy with the prior art quench method.

FIG. 2 shows the decrease of the ZT-value during thermal cycling. In theright hand side the curves represent from top to bottom: first heatingcycle, first cooling cycle, second heating cycle, third heating cycle,second cooling cycle, third cooling cycle.

FIGS. 3 a-3 b 1 show the distribution of Seebeck coefficients of a zonerefined Zn₄Sb₃ sample where the pulling speed during zone refinement ofthe sample was too high. The sample subjected to zone; refinement wasmade by a process in analogy with the prior art quench method.

FIGS. 4 and 4 a show the distribution of Seebeck coefficients of a zonerefined Zn₄Sb₃ sample where the pulling speed during zone refinement ofthe sample was correct. The samples subjected to zone refinement wasmade by a process analogy with the prior art quench method.

FIGS. 5-5 b show the distribution of Seebeck coefficients or a zonerefined Zn₄Sb₃ sample where the pulling speed during zone refinement ofthe sample was correct. The sample subjected to zone refinement was madeby a process in analogy with the prior art quench method.

FIGS. 6-6 c show the distribution of Seebeck coefficients of a zonerefined Zn₄Sb₃ sample doped with Mg.

FIG. 7 shows an x-ray powder diagram of a zone refined Zn₄Sb₃ samplewhere the pulling speed during zone refinement of the sample was toohigh. The sample subjected to zone refinement was made by a process inanalogy with the prior art quench method.

FIG. 8 shows an x-ray powder diagram of a zone refined Zn₄Sb₃ samplewhere the pulling speed during zone refinement of the sample was toolow. The sample subjected to zone refinement was made by a process inanalogy with the prior art quench method.

FIGS. 9 a-9 d show physical data of a quenched (lower curve) and a zonerefined (upper curve) Zn₄Sb₃ sample respectively which have both beenthermally cycled. The Zn₄Sb₃ quenched material was made by a process inanalogy with the prior art quench method.

FIGS. 10-10 b show the distribution of Seebeck coefficients of aquenched Zn₄Sb₃ sample doped with Cd.

FIGS. 11-11 b show the distribution of Seebeck coefficients of aquenched Zn₄Sb₃ sample doped with Mg.

FIG. 12 illustrates the improved properties in terms of “loss” of figureof merit in respect of a zone refined Zn₄Sb₃ sample compared to a Zn₄Sb₃sample which has not been zone refined. Curve 1 a represents a Zn₄Sb₃sample before thermal cycling and curve 1 b represents the same sampleafter thermal cycling. The sample was made in analogy with a prior artquench method. Curve 3 a and 3 b represent a zone refined Zn₄Sb₃ samplemeasured before and after thermal cycling respectively, and curve 2 aand 2 b represent a zone refined Zn₄Sb₃ sample in which additional Znwas added during hot pressing, measured before and after thermal cyclingrespectively

FIG. 13 shows an x-ray powder diagram of a quenched Zn₄Sb₃ sample madeby a process in analogy with the prior art quench method.

FIG. 14 shows an x-ray powder diagram of a zone refined Zn₄Sb₃ sample.

FIGS. 15 a-15 d show physical data of a zone refined Zn₄Sb₃ sample whichhas not been thermally cycled. The sample subjected to zone refinementwas made by a process in analogy with, the prior art quench method.

FIG. 16 shows an x-ray powder diagram of a zone refined Mg-doped Zn₄Sb₃sample.

FIG. 17. shows the set-up for measuring Seebeck coefficients of athermoelectric sample.

FIG. 18 shows the phase diagram of a material having the composition ABand having a phase diagram showing a peritectic reaction.

DETAILED DESCRIPTION OF THE INVENTION

The Inventive Thermoelectric Material

It has now been found that the above mentioned reduction in the figureof merit under thermal cycling at least partly can be assigned to degreeof phase impurities in the thermoelectric material.

According to one aspect of the present invention a thermoelectricmaterial of the p-type having the stoichiometric formula Zn₄Sb₃, whereinpart of the Zn atoms optionally being substituted by one or moreelements selected from the group comprising Sn, Mg, Pb and thetransition metals in a total amount of 20 mol % or less in relation tothe Zn atoms, is provided.

The thermoelectric materials of the p-type according to the presentinvention are in terms of ZT quality stable after thermal cycling; andsaid materials exhibit a figure of merit, ZT of 0.5 or higher at 350° C.and/or of 0.6 or higher at 400° C.

The materials of the present invention have turned out to exhibit a highdegree of phase purity in the sense that compared to prior artmaterials, the materials according to the present inventions does notcontain and/or are less prone to form inclusions of the undesiredZnSb-phase.

One way of characterising the phase homogeneity of a thermoelectricmaterial is by expressing the homogeneity in terms of a spatial Seebeckmicroprobe scan. The inventive materials according to the presentinvention may be characterised in that the material exhibits a singlepeak in a spatial Seebeck microprobe scan. The homogeneity of thematerial can be characterised via measures of the width of the singlepeak of the best fitted curve as expressed in the equation (i) below:y=y ₀+(A/(w*(π/2)^(1/2)))*exp(−2*((x−xc)/w)²)  (i)(see section below for an explanation as to the various factorsappearing in equation (i)).

Accordingly, in the present application the width, w is the width of thepeak at maximum S/√e, where e is the exponential coefficient, and thematerials according to the invention may be characterised in that saidmaterials have a homogeneity as expressed by the width, w of the peak atmaximum S/√e of the best fitted curve of a spatial Seebeck scanning of15 μVK⁻¹ or less. In the present description and in the appended claimsthe term “width, w of the peak at maximum S/√e” means width of the peakat the position of the height of the peak divided by √e. The reason forusing this measure is that it is an appropriate measure derived from thebest fitted Gauss-curve obtained from processing the measured data.

Hence in addition to x-ray powder diffraction analysis, the spatialSeebeck microprobe scan may present a useful method for evaluating on apreliminary basis, the usefulness of a potential thermoelectricmaterial.

As set out above, the inventive thermoelectric material in addition ofbeing of the stoichiometric formula Zn₄Sb₃, also comprises material ofthe stoichiometric formula Zn₄Sb₃ wherein part of the Zn atoms aresubstituted by one or more elements selected from the group comprisingSn, Mg, Pb and the transition metals in a total amount of 20 mol % orless in relation to the Zn atoms of Zn₄Sb₃.

The elements referred to as “transition elements” in the presentdescription and the appended claims are to be understood as the groupcomprising the following elements: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir, Pt,Au, Hg, and Ac.

The positive effect of doping a transition metal into the compound is atleast two-fold. Firstly, it will lower the lattice thermal conductivityby introducing disorder in the structure. Secondly, it may introduced-bands from the transition metal just below the Fermi level. Anasymmetric band structure near the Fermi level leads to an increasedSeebeck coefficient.

However, whether the thermoelectric materials according to the presentinvention are doped or not, it is important that they exhibit a phasediagram showing a peritectic reaction: Preferably they exhibit a phasediagram showing a peritectic reaction analogue to Zn₄Sb₃.

It should be noted that in the present application and in the appendedclaims, the term “a material having the stoichiometric formula Zn₄Sb₃”is to be interpreted as a material having a stoichiometry whichtraditionally and conventionally has been termed Zn₄Sb₃ and having aZn₄Sb₃ crystal structure. However, it has recently been found that thesematerials having the Zn₄Sb₃ crystal structure contain interstitial zincatoms making the exact stoichiometry Zn_(12.82)Sb₁₀, equivalent to thestoichiometry Zn_(3.846)Sb₃ (cf. Disordered zinc in Zn4Sb3 with PhononGlas, Electron Crystal Thermoelectric Properties, Snyder, G. J.;Christensen, M.; Nishibori, E.; Rabiller, P.; Caillat, T.; Iversen, B.B., Nature Materials 2004, 3, 458-463; and Interstitial Zn atoms do thetrick in Thermoelectric Zinc Antimonide, Zn₄Sb₃. A combined MaximumEntropy Method X-Ray Electron Density and an Ab Initio ElectronicStructure Study, Caglioni, F.; Nishibori, E.; Rabiller, P.; Bertini, L.;Christensen, M.; Snyder, G. J.; Gatti, C.; Iversen, B. B., Chem. Eur. J.2004, 10, 3861-3870). In the present application and in the appendedclaims the optional substitution of one or more elements selected fromthe group comprising Sn, Mg, Pb and the transition metals in a totalamount of 20 mol % or less in relation to the Zn atoms is based on theamount of Zn atoms of the exact stoichiometry Zn₄Sb₃. Accordingly, thestoichiometry of a material according to the present invention havingthe maximum degree of substitution of metal X is Zn_(3.2)X_(0.8)Sb₃.

In one preferred embodiment of the present invention the inventivethermoelectric material has the stoichiometric formula Zn₄Sb₃.

In another preferred embodiment of the thermoelectric material accordingto the present invention, a part of the Zn atoms of the material havingthe stoichiometric formula Zn₄Sb₃ has been substituted with Mg.

In a further and preferred embodiment of the thermoelectric materialaccording to the present invention, a part of the Zn atoms of thematerial having the stoichiometric formula Zn₄Sb₃ has been substitutedwith Cd.

In a still further and preferred embodiment of the thermoelectricmaterial according to the present invention, a part of the Zn atoms ofthe material having the stoichiometric formula Zn₄Sb₃ has beensubstituted with Hg.

In a yet still further and preferred embodiment of the thermoelectricmaterial according to the present invention, a part of the Zn atoms ofthe material having the stoichiometric formula Zn₄Sb₃ has beensubstituted with Pb.

In another and preferred embodiment of the thermoelectric materialaccording to the present invention, a part of the Zn atoms of thematerial having the stoichiometric formula Zn₄Sb₃ has been substitutedwith Sn.

In yet a still further and preferred embodiment of the thermoelectricmaterial according to the present invention, a part of the Zn atoms ofthe material having the stoichiometric formula Zn₄Sb₃ has beensubstituted with Mg; and a part of the Zn atoms has been substitutedwith Cd.

It should be understood that in the present application, when thethermoelectric material according to the present invention has thestoichiometric formula Zn₄Sb₃ wherein part of the Zn atoms issubstituted by one or more elements selected from the group comprisingSn, Mg, Pb and the transition metals, the amount of the totalsubstitution may be 20% or less, such as 19% or less, e.g. 18% or less,for example 17% or less, or 16% or less, such as 15% or less, e.g. 14%or less, for example 13% or less, or 12% or less, such as 11% or less,e.g. 10% or less, for example 9% or less, or 8% or less, such as 7% orless, e.g. 6% or less, for example 5% or less, or 4% or less, such as 3%or less, e.g. 2% or less, for example 1% or less, or not more than 0.9%,0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2% or 0.1%; all percentages beingmol %.

Furthermore, it should be understood, that when the thermoelectricmaterial according to the present invention has the stoichiometricformula Zn₄Sb₃, wherein part of the Zn atoms is substituted in an amountof any of the above-mentioned percentages, the substitution may compriseany one or any combination of one or more elements selected from thegroup comprising Sn, Mg, Pb and the transition metals. Additionally, itshould be understood that when the substitution comprises two or moreelements selected from the group comprising Sn, Mg, Pb and thetransition metals, the mutual molar amount of these dopant elements maybe of any ratio.

Hence, the thermoelectric materials of the present invention embracesZn₄Sb₃ and any combination of substitution of part of the Zn atoms ofone or more elements selected from the group comprising Sn, Mg, Pb andthe transition metals in any mutual molar ratio and in a total amount ofsubstitution of any of the above percentages.

By the process for the manufacture of said thermoelectric materialsaccording to the present invention it has been possible to obtain theabove materials, in which the degree of phase purity is very high; i.e.the obtained inventive materials show only minor variations as to thepossible various phases of the material.

The process according to the present invention for the manufacture ofthe inventive thermoelectric materials is a so-called zone meltingprocess in which an interphase of two materials having differentstoichiometries is melted and in which the obtained melted zonesubesequently is moved in relation to the material so as to obtain thesolidified material in a high phase purity.

It has been found that the phase purity of the thermoelectric materialhas a tremendous impact on the decrease of the ZT-value during thermalcycling. Thus, when obtaining thermoelectric materials according to thepresent invention which have a high degree of phase purity, the “loss”of the figure of merit, the ZT-value, during thermal cycling, isconsiderably reduced as compared to materials which have not beenmanufactured by a zone melting process.

The reason for this is not fully understood, but it is believed that thehigh purity and homogeneity prevents growth of undesired impurity phasesdue to the fact that only minor amounts of crystal “seeds” of undesiredcrystal structure are present in a material having a high degree ofphase purity. A lower content of undesired crystal structures may makethe material less prone to undergo a phase transition during heating andcooling cycles.

FIG. 1 a, FIG. 1 b and FIG. 2 show the effect of thermal cycling of aZn₄Sb₃ material on the Seebeck coefficient, S, the electricalconductivity, σ, and ZT-values respectively. The material was thermallycycled within the temperature range of 50-350° C. It can be seen thatwhen the material has been thermally cycled a few times, somewhat stablevalues are obtained. In FIG. 1 a the lower curve corresponds to thefirst part of the heating cycle, and in FIG. 1 b the upper curvecorresponds to the first part of the heating cycle. In the right handside of FIG. 2 the curves represent from top to bottom: first heatingcycle, first cooling cycle, second heating cycle, third heating cycle,second cooling cycle, third cooling cycle.

FIGS. 15 a-15 d show the physical properties of a zone refined Zn₄Sb₃material according to the present invention prior to thermal cycling.The sample subjected to zone refinement was made by a process in analogywith the prior art quench method. FIGS. 15 a-d show that a figure ofmerit of above 1.1 can be obtained at approximately 400° C. and FIGS. 9a-d show the physical properties of a zone refined Zn₄Sb₃ materialaccording to the present invention (upper curve) after thermal cycling.FIGS. 9 a-d show that a figure of merit of approximately 0.675 can beobtained at approximately 350° C.

Comparing FIGS. 15 a-15 d with FIGS. 9 a-9 d illustrates the “loss” offigure of merit during thermal cycling. FIGS. 9 a-d indicate that the“loss” of figure of merit is smaller for a zone refined sample comparedto a sample made by a prior art quench method. The same property of“loss” of figure of merit during thermal cycling is also illustrated inFIG. 12. FIG. 12 shows that the “loss” of figure of merit during thermalcycling in respect of a Zn₄Sb₃ material that was made in analogy with aprior art quench method is many times larger than the “loss” of figureof merit during thermal cycling in respect of a Zn₄Sb₃ material that wassubjected to zone refinement (compare difference of curve 1 a (quenchedsample prior to thermal cycling) and 1 b (quenched sample after thermalcycling) with that of curve 3 a (zone refined sample prior to thermalcycling) and 3 b (zone refined sample after thermal cycling) or that ofcurve 2 a (zone refined sample with added Zn during hot pressing priorto thermal cycling) and 2 b (zone refined sample with added Zn duringhot pressing after thermal cycling).

The high degree of phase purity of the inventive thermoelectricmaterials has been confirmed by the above-mentioned spatial Seebeckmicroprobe scanning method (or spatial Seebeck scan for short) which isfurther described below.

Bearing in mind that the Seebeck-value, S is defined as dV/dT, whereindV is the potential difference present on the material, and dT is thetemperature difference present at the position of the material and thatthe Seebeck-value accordingly is related to the ability of athermoelectric material to produce a potential difference when arrangedin a temperature gradient, it is implied that a useful thermoelectricmaterial must have a high S-value.

In a spatial Seebeck scanning method a cross-section of thethermoelectric material is scanned so as to obtain specificSeebeck-values for very small areas of the cross-section of thematerial. By plotting the distribution of Seebeck-values—i.e. byplotting the number of times a specific Seebeck-value is measured as afunction of each specific Seebeck-value—the phase purity can bevisualised.

If the obtained curve shows a single, sharp peak a vast majority of eachsmall scanned area exhibits nearly the same Seebeck-value which isindicative of a high phase purity. If, on the other hand, the curveshows two or more peaks and/or if each peak is not very sharp, thescanned area of the thermoelectric material seems to be not veryhomogeneous as to the distribution of Seebeck-values, which isindicative of a high degree of phase impurity.

It should be noted however, that in the present application the term“peak” of a spatial Seebeck scan is to be interpreted as the peak of thebest-fitted curve of the distribution of the Seebeck-values measured.This curve is obtained by fitting each accumulated Seebeck value to thefollowing gauss equation:y=y ₀+(A/(w*(π/2)^(1/2)))*exp(−2*((x−xc)/w)²)  (i)

-   -   wherein y₀ usually is equal to 0; w is the width; x is Seebeck        value, xc is peak value (=Smedium), and A is the area of the        curve

Accordingly, a spatial Seebeck scan may be an easy way to assess on apreliminary basis the quality of a thermoelectric material in terms ofSeebeck-values, and it has been found that the inventive thermoelectricmaterials according to the present invention which are manufactured bythe zone melting process according to the present invention exhibit asingle, sharp peak in a spatial Seebeck scan having a homogeneity of 15μVK⁻¹ or less as expressed by the width, w of the peak at maximum S/√e.

Furthermore the inventive thermoelectric materials according to thepresent invention which are manufactured by the zone melting processaccording to the present invention has improved behaviour in terms ofreduction or “loss” of ZT-value during thermal cycling, as compared tothermoelectric materials which has not been made by a zone meltingprocess.

FIGS. 9 a-9 d show physical data measured on a Zn₄Sb₃ material which hasnot been zone refined (a quenched material made by a process in analogyto the prior art quench method); and a zone refined Zn₄Sb₃ materialaccording to the present invention respectively. The upper curvecorresponds to the zone refined material. Both samples have beenthermally cycled until constant values of properties were obtained. Inrespect of the thermal conductivity measurements the two curves areessentially superimposed. FIGS. 9 a-9 d show that zone refining enhancesthe figure of merit, ZT. At 350° C. the ZT-value is increased from 0.4without zone refining to 0.675 when zone refined. This corresponds to anincrease of approximately 0.68%.

As it appears from the above, the distribution of Seebeck-values, asobtained by a spatial Seebeck scan may in fact be utilised as aqualitative measure of the phase purity of the material. Accordingly,the distribution of the Seebeck-values in the Seebeck scan can be usedfor defining a given material.

Preferably, the inventive thermoelectric materials according to thepresent invention which are manufactured by the zone melting processaccording to the present invention exhibit a single, sharp peak in aspatial Seebeck scan having a homogeneity of 15 μVK⁻¹ or less asexpressed by the width, w of the peak at maximum S/√e.

More preferred, the inventive thermoelectric materials exhibit a singlepeak corresponding to S≧90 μVK⁻¹. By the term “corresponding to S≧90μVK⁻¹” it is understood that the top of the single peak corresponds to avalue of S of not less than 90 μVK⁻¹ in a spatial Seebeck microprobescan. Preferably the inventive thermoelectric materials exhibit a singlepeak corresponding to S≧100 μVK⁻¹, such as S≧110 μVK⁻¹.

It should be noted that in this application, when reference is made to aspatial Seebeck scan of a sample, it is understood that the sample is“pure” in terms of conventional x-ray powder diffraction analysis, i.e.that the total volume fraction of crystalline impurity phases is belowquantification level (typically below 1-2%).

FIGS. 5-5 b visualise the phase purity of a zone refined Zn₄Sb₃ materialaccording to the present invention. The sample was scanned along thelengthwise direction. The curve in the upper part of FIGS. 5-5 b showsthe distribution or Seebeck values in respect of specific valuesmeasured, and in the plot in the lower part of FIGS. 5-5 b thedistribution is visualised by way of shade plotting; each shadecorresponds to a specific S value. The homogeneity as expressed by thewidth, w was found to be 13.7 μVK⁻¹.

Similarly, FIGS. 6-6C visualise the phase purity of a zone refinedZn₄Sb₃ material doped with 1% Mg according to the present invention. Thesample was scanned along the lengthwise direction. The homogeneity asexpressed by the width, w was found to be 5.7 μVK⁻¹.

FIGS. 10-10 b visualise the phase purity of a quenched Zn₄Sb₃ materialdoped with 0.1% Cd according to the present invention. The sample wasscanned along the lengthwise direction. The homogeneity as expressed bythe width w was found to be 6.7 μVK⁻¹.

The Spatial Seebeck Scanning Method

The following section explains how to perform the spatial Seebeck scan.

The Seebeck coefficient is an indirect measure for charge carrierconcentration and thus gives information about the components of theinvestigated material.

A scanning Seebeck micro-thermoprobe is a device for measuring theSeebeck coefficient on a sample's surface spatially resolved, and asexplained above this provides information as to the homogeneity ordistribution of the components.

Measuring Principle in the Seebeck Microprobe:

With reference to FIG. 17 the microprobe for use in a spatial Seebeckscan is explained below.

A sample is divided into two halves by cutting the sample in e.g. alengthwise direction using a diamond wire saw.

A heated probe tip (50) is positioned onto the surface of a sample (30),which due to the cutting is exposed. The probe is connected with athermocouple (40) (in this case a Cu—CuNi type) measuring thetemperature T₁. The sample is in good electrical and thermal contactwith a heat sink (20) and also connected with a thermocouple measuringT₀. The probe tip (50) heats the sample in the vicinity of the tipleading to a temperature gradient. Tip (50) as well as heat sink (20) istemperature controlled and can be moved via linear stages.

Combining the Cu—Cu and the CuNi—CuNi wires of the thermocouples,voltages U₀ and U₁ are measured, resulting in the following equations:U ₀=(S _(S) −S _(Cu))·(T ₁ −T ₀); andU ₁=(S _(S) −S _(CuNi))·(T ₁ −T ₀);giving:

${S_{s} = {{\frac{U_{0}}{U_{1} - U_{0}}( {S_{Cu} - S_{CuNi}} )} + S_{cu}}},$

S_(s) being the Seebeck coefficient of the sample at the position of theprobe tip.

A major prerequisite of the measuring principle is to keep a temperaturegradient between the probe and the sample. A suitable value for thegradient turns out to be around 3 K.

Three linear stages are used to scan the surface in the x-y plane (10)of the sample with a physically limited resolution of up to 10 μmdepending on the thermal conductivity of the scanned material. Toachieve the required high resolution, the sample must have good thermalcontact to the thermocouple and a heat sink. In this case it is embeddedinto a low melting solder (Wood-metal) with the thermocouple.

For additional details relating to the spatial Seebeck scan, referenceis made to: P. Reinshaus, H. Süβmann, M. Bohm, A. Schuck, T. Dietrich,Proc. of the 2nd European Symposium on Thermoelectrics—Materials,Processing Techniques and Applications, Dresden, Germany 1994, 90, whichis hereby incorporated as reference.

Although the Seebeck measurements may depend on the calibration of agiven apparatus, and thus the absolute values may vary, the relativevalues of the Seebeck coefficients measured for different samples on thesame equipment are reliable. Because the width, w of the peak of theSeebeck scan may be used as a parameter for defining the phase purity,it is desirably that, the obtained width, w is not instrument-dependent.In fact, it has turned out, that the empirically obtained width, w willshow little dependence on the exact calibration of the instrument. Inother words the width w is independent on the exact absolute valuesmeasured for a given apparatus—it represents a distribution of values.

Obtaining the Figure of Merit Empirically

As set forth above, the efficiency of any thermoelectric material ischaracterised by the figure of merit,ZT=S ² σT/κ,where S is the Seebeck coefficient, σ the electrical conductivity and κthe thermal conductivity and T is the absolute temperature.

In this work, when obtaining the figure of merit, all the factorsappearing in the right hand side of the above equation were measuredseparately.

A—Measuring the Temperature Dependent Seebeck Coefficient S

A sample was placed into an oven and contacted with two thermocouples(Pt/PtRh) that have a distance of about 8 mm. One side of the sample iscontacted with a heat sink so that a temperature gradient is created.This temperature gradient leads to the measured thermopower U(T).Derivating U(T) to the temperature gives the Seebeck coefficient at thetemperature T. The thermopower was measured in a range of differenttemperatures.

B—Measuring the Electrical Conductivity σ

A rectangular sample was placed into an oven and contacted at bothopposite faces with an AC current source. One side perpendicular tothese contacts is contacted with two potential probes of tungstencarbide. The measured voltage is proportional to the electricalconductivity and the current applied to the sample and a geometricalcorrection factor. The voltage thus giving the electrical conductivitywas measured in a range of different temperatures.

C—Measuring the Thermal Conductivity κ

The thermal conductivity κ is proportional to the density ρ, the thermaldiffusivity λ and the specific heat C_(p), κ=ρλC_(p).

λ was measured with a commercial laser flash apparatus, C_(p) wasmeasured with a commercial DSC (differential scanning calorimeter) andthe density results from the weight and the geometric dimensions. Theseparameters which give the thermal conductivity according to the equationgiven above were measured in a range of different temperatures.

All the measured parameters given above were applied according to theequation:ZT=S ² σT/κ,

-   -   in order to express the figure of merit as a function of        temperature.

Processes for the Manufacture of the Inventive Thermoelectric Materials

A—Zone Melting Process

The improved thermoelectric materials according to the present inventionwere manufactured by a zone melting process.

In this process an interphase between a “non-stoichiometric”material—i.e. a material which does not obey the stoichiometric formulaof the desired thermoelectric material and a “stoichiometric” materialhaving the stoichiometric formula of the desired material, is created soas to form an “arrangement”, and subsequently a heating zone isestablished near or preferably around a part of the “non-stoichiometric”material so as to cause a section of the “non-stoichiometric” materialto melt, whereafter the position of the melted material is moved inrelation to the materials making up the arrangement in order to obtainthe final product.

The interphase is preferably provided in the form of a rod of the“non-stoichiometric” material, and a “feeding rod” comprising athermoelectric material having the stoichiometric formula which isdesired for the final thermoelectric product. Preferably the arrangementis obtained by arranging one of the materials on top of the other.

In the present application the term “rod” is to be interpreted as anykind of geometrical shape which is suitable for the intended purpose,i.e. suitable for forming either the “non-stoichiometric” or the“stoichiometric” material to be melted in the zone refining processaccording to the present invention.

Subsequently the interphase between the two types of material issubjected to a zone melting, for example by placing this arrangement inthe centre of an induction furnace. It is however preferred first toenclose the arrangement in a vacuum. This is preferably achieved byplacing the arrangement in an enclosure, such as a quarts tube whichsubsequently preferably is evacuated and closed by melting its ends,thereby forming an ampoule. In a preferred embodiment the“non-stoichiometric” material is surrounded by two “feeding rods”thereby forming two interphases between different materials. In thisembodiment—when the arrangement is arranged in an up-rightconfiguration—the lower “feeding rod” simply acts as a support for thearrangement.

Subsequently the zone melting is started by applying power to thefurnace so that the heat produced at a position corresponding to theinterphase between the “nonstoichiometric” material and one feeding rodmelts the material of said interphase.

When melting has been observed for about 15 min the power is slightlylowered, e.g. over a time span of approximately 10 min. After anadditional time span of approximately 10 min., the ampoule is movedthrough the heating zone of the furnace at a very low speed, such as0.6-1.8 mm/h. In this way the melting zone travels through the materialat the same speed. When the melting zone has passed through the entirearrangement, the power is switched off and the arrangement is allowed tocool.

It has been found that when the speed of movement of the melting zone isapproximately 1.0-1.5 mm/hour, the obtained zone-melted materialsexhibits very high phase purities and high ZT-values as well as low“loss” of ZT-value during thermal cycling.

At this stage the part(s) of the arrangement originating from the“feeding rod(s)” is/are cut off and the remaining part is hot-pressed asset out below.

Since the crystals grown by melting and purified by zone refinement arebrittle, samples were reduced to small sizes, ball milled and then hotpressed with a hot uniaxial press (HUP). The HUP consists of a vacuumchamber with an oven and a hydraulic system. Powder is filled into apressing die of graphite that is mounted between the hydraulic plungersin the oven. The temperature and the pressure as well as thetranslational displacement of the plungers are controlled.

Bulk samples were prepared from powders using the hot uniaxial press(HUP). Since consolidation parameters are strongly dependent on thespecific material, several tests have been performed to discover themost suitable temperature, pressure and duration for each material. Aseries of Zn₄Sb₃ samples was ground and hot-pressed at differentcombinations of temperature and pressure to investigate the influence ofconsolidation conditions on the thermoelectric properties.

Thus, a program of material consolidation has been performed to obtainthe most suitable pressure, temperature and duration, which was found tobe 100 MPa, 370° C. and 30 min under an inert atmosphere of 500 hPanitrogen. All materials were ball-milled in hexane and hot pressed undervacuum. It has turned out that variations in the hot pressing parametersmay account for variations in the quality of the resultingthermoelectric materials.

In some cases it was found that zinc was lost during pressing or heattreatment (probably by evaporation). In such cases it has turned outthat adding zinc powder prior to HUP pressing improves thethermoelectric properties of the materials. Zinc powder—if added—wasadmixed in an amount of 1.2 atom % with the crushed Zn₄Sb₃ sample andthis mixture was then ball-milled (300 rpm) in hexane for one hour andsubsequently subjected to the HUP pressing using the above parameters.

FIG. 12 shows the figure of merit of thermally cycled Zn₄Sb₃ materials.Zone refining (curve 3 b) improves the properties of a quenched (curve 1b) sample. Addition of zinc prior to the HUP (curve 2 b) furtherincreases the figure of merit.

The size of the final samples was varied between 16 mm in diameter and 1mm in thickness for thermoelectric measurements and evaluation and 4 mmin diameter and 3-5 mm in thickness for the module preparation. Alsosamples with 16 mm diameter and 3 mm thickness have been prepared formodule fabrication. However, the cutting of the samples may be difficultdue to the brittleness of the material. 4 mm diameter samples can beused directly for the module.

The importance of the choice of pulling speed in the zone refiningmethod according to the present invention are illustrated by FIGS. 3 a,3 b, 4, 5, 7, 8 and 14:

FIG. 3 a shows the phase purity of a zone refined Zn₄Sb₃ material inwhich the pulling speed during zone refinement was too high. The samplewas scanned along the lengthwise direction. Besides of being impure interms of the various shades in the shade plotting, the sample also showsimpurity in terms of exhibiting two peaks in the scan, suggesting thepresence of two different phase structures.

FIG. 3 b shows the phase purity of the same sample. The sample wasscanned in two dimensions of a cross section.

Similarly, FIG. 4 shows the phase purity of a zone refined Zn₄Sb₃material in which the pulling speed during zone refinement was too low.

In contrast, FIGS. 5 a-5 d show the phase purity of a zone refinedZn₄Sb₃ material in which the pulling speed during zone refinement wascorrect (1.2 mm/hour). The sample was scanned along the lengthwisedirection. The sample has a purity as expressed by the width, w of thepeak at maximum S/√e of 13.7 μVK⁻¹.

FIG. 7 and FIG. 8 are x-ray powder diffraction diagrams of zone refinedZn₄Sb₃ materials in which the pulling speed during zone refinement wastoo high and too low respectively (the continuous spectrum shows themeasured data). The lines in the diagram correspond to the ZnSb impurityphase implying that too high and too low pulling speeds respectively inthe zone refining methods result in ZnSb-phases in the final material.

In contrast FIG. 14 is an x-ray powder diffraction diagram of a zonerefined Zn₄Sb₃ material in which the pulling speed during zonerefinement was correct (the continuous spectrum shows the measureddata). The lines in the diagram correspond to the Zn₄Sb₃ phase, andcorrespondence between the lines and the continuous spectrum suggeststhat a correct pulling speed in the zone refining method results in thecorrect Zn₄Sb₃ phase in the final material. The sample of FIG. 14 wasmade by a process in analogy with the prior art quench method followedby zone refinement and hot pressing.

B—Preparation of the “Feeding Rods” for Use in the Zone Melting Process

Process for the Manufacture of “Feeding Rods”

According to one aspect of the present invention the “feeding rods” foruse in the zone melting process may be obtained by a simple thermalquench process in analogy with the prior art quench method (cf. Caillatet al., J. Phys. Chem. Solids, Vol. 58, No 7, pp. 1119-1125, 1997).

In this process Zn pellets, Sb and optionally substituents (selectedfrom the group comprising Mg, Sn, Pb and the transition metals) in thedesired mutual molar ratio, i.e. the ratio which is desired for thefinal thermoelectric material, are mixed and placed in an enclosure,such as a quartz tube. Preferably the Zn pellets prior to mixing withthe other components are rinsed in dilute acid, such as dilute HCL, forexample 4M HCl, whereafter they preferably are rinsed in water followedby ethanol and dried with e.g. a hair dryer. Subsequently the tube isevacuated and closed by melting, thereby resulting in a quarts ampoule.The tube is evacuated to a final pressure of 10⁻⁵ bar or less, or 10⁻³bar or less. The obtained ampoule is then placed in a rotation deviceinside a furnace, such as a tube furnace. The furnace is switched on,and the ampoule is rotated so as to ensure mixing and homogenousheating. Preferably the ampoule is heated from room temperature toapproximately 700° C. at 200-400° C./h. After a few hours, such as 1-3hours, preferably 2 hours at this temperature, the ampoule is quicklyplaced in a vessel containing water for thermal quenching.

It has in fact been found, that when the quench process is carried outin respect of a material that has been doped with Mg and/or Cd andpossibly also with other dopants, such as Pb, according to thethermoelectric material according to the present invention,thermoelectric materials are obtained which exhibit excellent propertiesin terms of ZT-values and phase purity as expressed by the spatialSeebeck microprobe scanning, even without the zone melting process,which however will further improve the properties of these materials.The same is true in respect of undoped Zn₄Sb₃ material; albeit thismaterial when not zone refined exhibits a smaller degree of phase puritycompared to the doped materials.

Thus one aspect according to the present invention relates to a processfor the manufacture of a thermoelectric material according to thepresent invention comprising the steps necessary for the manufacture ofthe “feeding rods” followed by grinding and HUP pressing.

In a preferred embodiment such “quenched” materials has thestoichiometric formula Zn₄Sb₃, wherein part of the Zn atoms optionallybeing substituted by one or more elements selected from the groupcomprising Mg and Pb in a total amount of 20 mol % or less in relationto the Zn atoms

FIG. 13 is an x-ray powder diffraction diagram of a quenched Zn₄Sb₃material which has not been zone refined (the continuous spectrum showsthe measured data). The lines in the diagram correspond to the Zn₄Sb₃phase and the correspondence between the lines and the continuousspectrum suggests that although the sample has not been zone refined itnevertheless comprises the correct Zn₄Sb₃ phase.

FIGS. 9 a-9 d show the effect on physical properties of zone refining(upper curve) and a quenched Zn₄Sb₃ material made by a process inanalogy with the prior art quench method (lower curve). Both sampleshave been thermally cycled until constant values of properties wereobtained. In respect of the thermal conductivity measurements the twocurves are essentially superimposed. It is seen that zone refiningprocess improves the thermoelectric properties of the material.

FIGS. 10-10 b show the phase purity of a quenched Zn₄Sb₃ material whichhas been doped with 0.1 mol % cadmium. The sample was scanned along thelengthwise direction. Even though this material had not been zonerefined, it exhibits excellent phase purity as evidenced by a width, wof 6.7 μVK⁻¹.

Similarly, FIGS. 11-11 b shows the phase purity of a quenched Zn₄Sb₃material which has been doped with 0.1 mol % magnesium. The sample wasscanned along the lengthwise direction. Even though the material had notbeen zone refined, it exhibits an excellent phase purity expressed bythe width, w of 12.0 μVK⁻¹.

C—Preparation of the “Non-Stoichiometric” Materials for Use in the ZoneMelting Process

According to one embodiment of the present invention, the“non-stoichiometric” materials may be obtained by mixing the appropriateamounts of Zn and Sb respectively, then placing this mixture in anenclosure, such as a quartz tube, evacuating and closing said enclosurethereby forming an ampoule and placing the obtained ampoule in afurnace, such as a tube furnace, whereafter the material is sintered.

The mixing of the components is preferably performed by rotating thecomponent Zn and Sb in a flask for approximately 1 hour.

Preferably the mixture is sintered for approximately six hours. Morepreferred the material is sintered for more than approximately 12 hours.It appears that a longer sintering time results in a better mechanicalstability of the “non-stoichiometric” material. The sinteringtemperature is suitably selected in the range of 400-550° C., such asapproximately 400-450° C.

When sintering is completed the ampoule is broken and the material isused as is.

The molar ratio Zn to Sb for the “non-stoichiometric” composition ispreferably in the range 57:43 to 51:49, such as 56:44 to 52:48, forexample 55:45 to 53:47 such as 54:46. The most preferred molar ratio Znto Sb for the “non-stoichiometric” composition is approximately 52:48.

The enclosure containing the “non-stoichiometric” composition ispreferably evacuated to a final pressure of 10⁻⁵ bar or less, or 10⁻³bar or less.

The above inventive process presents an easy and time efficient way tothe manufacture of the improved thermoelectric materials according tothe present invention. As revealed by a spatial Seebeck scanning theobtained inventive materials having the Zn₄Sb₃ type structure exhibit asingle, sharp peak indicating a high degree of phase purity (cf. FIG. 5,FIG. 6, FIG. 10 and FIG. 11. Furthermore, the thermoelectric materialsaccording to the present invention exhibiting a single peak in saidspatial Seebeck scanning have been shown to have a Zn₄Sb₃ type structure(cf. FIG. 13, FIG. 14 and FIG. 16), And finally, as evidenced by FIGS.12 and 15, the thermoelectric materials according to the presentinvention have excellent thermoelectric properties.

Preferably the thermoelectric materials of the p-type having thestoichiometric formula Zn₄Sb₃, wherein part of the Zn atoms optionallybeing substituted by one or more elements selected from the groupcomprising Sn, Mg, Pb and the transition metals in a total amount of 20mol % or less in relation to the Zn atoms according to the presentinvention prior to thermal cycling have a ZT at 375° C. of 1.3 or more,such as 1.4 or more, for example 1.5 or more; or 1.6 or more, such as1.7 or more.

The thermoelectric material of the above type having the stoichiometricformula Zn₄Sb₃ according to the present invention prior to thermalcycling preferably have a ZT at 400° C. of 1.4 or more, such as 1.5 ormore, for example 1.6 or more, such as 1.7 or more.

When subjected to thermal cycling the thermoelectric materials of thep-type having the stoichiometric formula Zn₄Sb₃, wherein part of the Znatoms optionally being substituted by one or more elements selected fromthe group comprising Sn, Mg, Pb and the transition metals in a totalamount of 20 mol % or less in relation to the Zn atoms according to thepresent invention preferably have a ZT at 350° C. of 0.4 or more, suchas 0.5 or more, for example 0.6 or more, such as 0.7 or more; or 0.8 ormore, such as 0.9 or more, e.g. 1.0 or more.

The thermoelectric materials of the p-type having the stoichiometricformula Zn₄Sb₃, wherein part of the Zn atoms optionally beingsubstituted by one or more elements selected from the group comprisingSn, Mg, Pb and the transition metals in a total amount of 20 mol % orless in relation to the Zn atoms according to the present inventionafter thermal cycling preferably have a ZT at 400° C. of 0.5 or more,such as 0.6 or more, for example 0.7 or more, such as 0.8 or more; or0.9 or more, such as 1.0 or more, e.g. 1.1 or 1.2 or more; for example1.3 or more, such as 1.4 or more.

The Zone Melting Process in a General Perspective

In a general perspective the zone melting process may be applied in thephase refining of an already existing thermoelectric material showing aperitectic reaction analogue to Zn₄Sb₃.

Hence, the zone melting process may also be applied in situations inwhich an improved purity of an already existing thermoelectric materialis desired. In this case, the already existing thermoelectric materialis simply used as “feeding rod(s)” as generally described above and thealready existing thermoelectric material is then subjected to the zonerefining process.

In order to be able to determine a suitable composition of the“non-stoichiometric” material to be used in the zone melting process insuch a situation, the phase diagram of the composition of the materialof the “feeding rod” should be consulted.

FIG. 18 shows a phase diagram of a material having a binary composition(of element A and element B) and showing a peritectic reaction analogueto Zn₄Sb₃. The x-axis represents the composition ranging (from left toright) from pure A to pure B. The y-axis represent the temperature.

The composition of the material of the “non-stoichiometric” rod shouldbe selected so as to correspond to the projection of the liquidus curve“x-m” of FIG. 18 on the composition axis (the x-axis).

In the case where the thermoelectric material is a tertiary material (amaterial composed of three elements), the phase diagram isthree-dimensional, and the projection of the liquidus curve “x-m” willbe a two-dimensional surface, etc.

A person skilled in the art will be able to obtain information of thecomposition corresponding to a “liquidus” curve for any giventhermoelectric material showing a peritectic reaction

In a preferred embodiment according to the present invention the zonemelting process according to the present invention is performed on analready existing thermoelectric material having the composition CoSb₃ orFeSb₂. The phase diagram of these thermoelectric materials can be foundin M. Hansen, “The constitution of binary alloys”, McGraw-Hill BookCompany, New York, 1958; and R. D. Elliot, “The constitution of binaryalloys, first supplement”, McGraw-Hill, Inc. New York, 1965.

The Manufacture of a Thermocouple

In a special embodiment according to the present invention the obtainedthermoelectric material is used as the p-type thermoelectric in athermocouple. By cutting this p-type material in suitable sizes andarranging and connecting such a piece of the appropriate size togetherwith an n-type thermoelectric material, a thermocouple is obtained in away known per se. See for example “Frank Benhard; TechnischeTemperaturmessung; Springer Berlin, 2003; ISBN 3540626727”.

The Manufacture of a Thermoelectric Device Comprising One or MoreThermocouples

In a special embodiment according to the present invention one or moreof the obtained thermocouples is/are arranged in a way known per se inorder to obtain a thermoelectric device. See for example “Frank Benhard;Technische Temperaturmessung; Springer Berlin, 2003; ISBN 3540626727”.

Use of the Thermoelectric Device for Thermoelectric Purposes

In another aspect according to the present invention the obtainedthermoelectric device is used for thermoelectric purposes.

Such uses are well-known for a person skilled in the art ofthermoelectrics.

EXAMPLES

In the following examples the chemicals used were:

Zn, 3-8 mm pellets (Merck 8780);

Sb 325 mesh 99.5% (Alfa Caesar 10099);

Mg “shavings” 99.98% Alfa Caesar 36193

Cd, “shavings”, 99.999% Cominco, Canada

Pb, granules, BDH 29014

Hg, superpur, Merck 4404

Sn, granules, 4 mm, Merck 7806

Example 1 Synthesis of a Zn₄Sb₃ Thermoelectric Material by ThermalQuenching in Analogy to the Prior Art Quench Method

Small pellets of Zn were cleaned a few seconds in 4M HCl, andsubsequently rinsed first in water and then in ethanol. They were driedwith a hair dryer. The Zn pellets were then weighed (M_(Zn)=22.83638 g),and placed in a quartz tube (inner diameter=12.5 mm, outer diameter=15mm). Additionally, antimony was added (M_(Sb)=31.89701 g). The tube wasevacuated to 10⁻⁵ bar and closed by melting resulting in a quartzampoule.

This ampoule was placed in a rotation device inside a tube furnace(HEREUS, ROK/A 6/30), and it was rotated consistently during heating soas to ensure mixing and homogenous heating. It was heated from roomtemperature to 700° C. at 400° C./h. After 2 hours the ampoule wasquickly placed in a water container for thermal quenching, therebyobtaining a rod.

FIGS. 9 a-9 d show physical data measured on the Zn₄Sb₃ material whichhas not been zone refined (the quenched material)—the lower curve. Thesample has been thermally cycled until constant values of propertieswere obtained.

FIG. 13 is an x-ray powder diffraction diagram of quenched Zn₄Sb₃material (the continuous spectrum shows the measured data). The lines inthe diagram correspond to the Zn₄Sb₃ phase and correspondence betweenthe lines and the continuous spectrum suggests that it comprises thecorrect Zn₄Sb₃ phase.

Example 2 Preparation of Mg-Doped Thermoelectric Materials

A range of different Mg-doped materials was made.

Following the procedure described in example 1 with the exception ofsubstituting part of the Zn with Mg: M_(Zn)=7.05042 g, M_(Mg)=0.05406 g,M_(Sb)=10.04673 g, a 2% Mg-doped thermoelectric material is obtained.

Similarly, a 0.1% Mg doped Zn₄Sb₃ material was made. FIG. 11 shows thephase purity of this thermoelectric material expressed by thedistribution of Seebeck values. The sample was scanned along thelengthwise direction. Even though this material had not been zonerefined, it exhibits a phase purity expressed by w of 12.0 μVK⁻¹.

Example 3 Preparation of Cd-Doped Thermoelectric Materials

Following the procedure described in example 1 with the exception ofsubstituting part of the Zn with Cd: M_(Zn)=4.13008 g, M_(Cd)=0.07172 g,M_(Sb)=5.82651 g, a 1% Cd-doped thermoelectric material is obtained.

Similarly, a 0.1% Cd doped Zn₄Sb₃ material was made. FIG. 10 shows thephase purity of this thermoelectric material expressed by thedistribution of Seebeck values. The sample was scanned along thelengthwise direction. Even though this material had not been zonerefined, it exhibits a phase purity expressed by w of 6.7 μVK⁻¹.

Example 4 Preparation of a 1% Hg-Doped Thermoelectric Material

Following the procedure described in example 1 with the exception ofsubstituting part of the Zn with Hg: M_(Zn)=9.74857 g, M_(Hg)=0.302 g,M_(Sb)=13.74376 g, a Hg-doped thermoelectric material is obtained.

Example 5 Preparation of a 1% Pb-Doped Thermoelectric Material

Following the procedure described in example 1 with the exception ofsubstituting part of the Zn with Pb: M_(Zn)=4.16204 g, M_(Pb)=0.13998 g,M_(Sb)=5.87129 g, a Pb-doped thermoelectric material is obtained.

Example 6 Preparation of a 1% Sn-Doped Thermoelectric Material

Following the procedure described in example 1 with the exception ofsubstituting part of the Zn with Sn: M_(Zn)=2.0523 g, M_(Sn)=0.0366 g,M_(Sb)=2.8981 g, a Sn-doped thermoelectric material is obtained.

Example 7 Preparation of a 1% Mg and 1% Cd-Doped Thermoelectric Material

Following the procedure described in example 1 with the exceptions ofsubstituting part of the Zn with Mg and Cd: M_(Zn)=8.43497 g,M_(Cd)=0.14284 g, M_(Mg)=0.03421 g, M_(Sb)=12.02077 g, a Mg- andCd-doped thermoelectric material is obtained.

Example 8 Synthesis of a Zn₄Sb₃ Thermoelectric Material by ZoneRefinement

A—Synthesis of a “Non-Stoichiometric” Material

Zn and Sb was weighed in desired stoichiometry of 52 mol % Zn and 48 mol% Sb and mixed by rotation for about 1 hour (M_(Zn)=16.19581 g,M_(Sb)=27.84659 g) The mixture was subsequently placed in a quartz tubehaving same inner diameter as the rod obtained in example 1. The tubewas evacuated to 10⁻⁵ bar and closed by melting. The resulting ampoulewas placed in a vertical tube furnace (HEREUS ROK/A 6/30) where it issintered for 12 hours at 400° C. and 14 hours at 450° C.

B—Zone Melting

In a quartz tube designed for mounting in an induction furnace 18 mm ofa rod obtained in example 1 (a “feeding rod”) was placed at the bottom.A 10 mm piece of the sintered “non-stoichiometric” material obtained asdescribed above was placed on top. Finally a large “feeding rod”, 70 mm,also prepared according to example 1 was placed on top of the“non-stoichiometric” rod. The quartz tube was closed by melting a pieceof glass at the upper part, which allows mounting on a vacuum pump. Thetube was evacuated to 10⁻⁵ bar and closed by melting. The quartz ampoulewas placed in a boron nitride (BN) holder at the bottom of the inductionfurnace (TSS model HP crystal growing furnace, 60 kW max power). The topof the ampoule was fastened inside another BN mount, which was screwedinto the pulling stick of the furnace. The ampoule was placed with theinduction coil positioned around the “non-stoichiometric” rod, i.e. thisis where the melt zone starts. The coil had three “windings” or coils inan overall conical shape. The top was 38 mm wide, the bottom 20 mm wide.Each coil was 5 mm thick. Heating was started and melting was observedin about 15 min., and the power was lowered slightly over 10 minutes.The power input was approximately 3.5 kW (2.5 kV, 1.4 A). After another10 minutes the ampoule was pulled through the induction coils at a rateof 1.2 mm per hour. The melted zone travelled through the coils with thesame speed, and the entire synthesis took 73 hours and 25 minutes.Subsequently the zone-refined Zn₄Sb₃ material was ball-milled underhexane and hot pressed at 100 MPa, 370° C. and 30 min under an inertatmosphere of 500 hPa nitrogen.

FIG. 5 shows the phase purity of this thermoelectric material expressedby the distribution of Seebeck values. The sample was scanned along thelengthwise direction. The material exhibits a phase purity expressed byw of 13.7 μVK⁻¹.

FIG. 15 shows physical data measured on the Zn₄Sb₃ material which hasbeen zone refined The measurements took place before thermally cycling.At approximately 400° C. the ZT-value is above 1.1.

FIG. 9 shows physical data measured on a Zn₄Sb₃ material which has notbeen zone refined (a quenched material); and a zone refined Zn₄Sb₃material according to the present invention respectively. The uppercurve corresponds to the zone refined material. Both samples have beenthermally cycled until constant values of properties were obtained. Inrespect of the thermal conductivity measurements the two curves areessentially superimposed. FIG. 9 shows that zone refining enhances thefigure of merit, ZT. At 350° C. the ZT-value is increased from 0.4without zone refining to 0.675 when zone refined which corresponds to anincrease of approximately 68%.

FIG. 14 is an x-ray powder diffraction diagram of the zone refinedZn₄Sb₃ material (the continuous spectrum shows the measured data). Thelines in the diagram correspond to the Zn₄Sb₃ phase and correspondencebetween the lines and the continuous spectrum suggests it comprises thecorrect Zn₄Sb₃ phase.

Example 9 Synthesis of a Mg-Doped Zn₄Sb₃ Thermoelectric Material by ZoneMelting

Following the procedure described in example 8 with the exceptions ofsubstituting part of the Zn with Mg: M_(Zn)=29.64571, M_(Mg)=0.11071 g,M_(Sb)=41.83186 g, a Mg-doped “feeding rod” was obtained.

FIG. 6 shows the phase purity of this thermoelectric material expressedby the distribution of Seebeck values. The sample was scanned along thelengthwise direction. The thermoelectric material exhibits a phasepurity expressed by w of 5.7 μVK⁻¹. FIG. 16 is an x-ray powderdiffraction diagram of the zone refined Mg-doped Zn₄Sb₃ material (thecontinuous spectrum shows the measured data). The lines in the diagramcorrespond to the Zn₄Sb₃ phase and correspondence between the lines andthe continuous spectrum suggests it comprises the correct Zn₄Sb₃ phase.

What is claimed is:
 1. A thermoelectric material of the p-type havingthe stoichiometric formula Zn₄Sb₃, wherein part of the Zn atoms aresubstituted by Mg in a total amount of 20 mol % or less in relation tothe Zn atoms.
 2. The thermoelectric material according to claim 1,wherein said material in respect of ZT quality as expressed by thefigure of merit, ZT, being stable after thermal cycling; and whereinsaid material exhibits a figure of merit, ZT of 0.5 or higher at 350° C.and/or of 0.6 or higher at 400° C.; said thermal cycling comprisingrepeatedly subjecting the material to consecutive temperature increasesand decreases within the temperature range of 50-350° C.
 3. Thethermoelectric material according to claim 1, wherein said material at350° C. exhibits a figure of merit, ZT of 0.6 or higher and/or whereinsaid material at 400° C. exhibits a figure of merit, ZT of 0.7 orhigher.
 4. The thermoelectric material according to claim 1, wherein thetotal amount of substitution of the Zn atoms is 18 mol % or less.
 5. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 16 mol % or less.
 6. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 14 mol % or less.
 7. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 12 mol % or less.
 8. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 10 mol % or less.
 9. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 8 mol % or less.
 10. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 6 mol % or less.
 11. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 4 mol % or less.
 12. Thethermoelectric material according to claim 1, wherein the total amountof substitution of the Zn atoms is 2 mol % or less.
 13. Thethermoelectric material according to claim 1, wherein said materialexhibits a single peak in a spatial Seebeck microprobe scan; and whereinsaid material has a homogeneity of 15 μVK−1 or less as expressed by thewidth, w of said peak at maximum S/√e of the best fitted curve of saidspatial Seebeck microprobe scan.
 14. The thermoelectric materialaccording to claim 13, wherein said material has a homogeneity of 13μVK−1 or less as expressed by said width, w of the peak at maximum S/√eof the best fitted curve of said spatial Seebeck microprobe scan. 15.The thermoelectric material according to claim 13, wherein said singlepeak corresponds to S≧90 μVK−1.
 16. A thermocouple comprising one ormore p-type thermoelectric materials according to claim
 1. 17. Athermoelectric device comprising one or more thermocouples according toclaim 16.